Chrony Data, Ballistic Calcs and Drops

[Gwion]

Hi All.

I've been trying to utilise the data i'm collecting to get some accurate calculations for drops at various ranges using JBM online ballistic calculator.

I am in putting accurate velocities from the chronograph. BC come from Brian Litz assessment of the 162gn Amax. Inputting accurate altitude, temperature, humidity, barometric pressure. Calculated dropas are SIGNIFICANTLY off real world scope settings. Also have accurate input for scope height, etc.

Anything i may be missing or screwing up that is giving me such wildly different results to the output calcs?

JBM is telling me 10.6MOA @ 500yd but i am actually 8.75MOA on the scope.
At 600yd, it was much cloaser to the calc. JBM said 14.2MOA, scope reads 13.75MOA.

While we're at it, can anyone explain the full 1moa difference between my first string at 500yd and the second string about 2 hours later? Second string had a full minute extra drop. I'm thinking it might have to do with my position behind the rifle giving me a slightly different line of sight through the scope; could there be a more environmental factor, though?

PS: maybe this should be in 'Target' section, but as i'm using it it to track load performance and development, i'm putting it here.

[brett1868]

If you post the data you're inputting I'll run it through a couple different ballistics apps I have and see what gets closest to actual.

Has the scope been calibrated? 40clicks doesn't always equal 10MOA as each click might be exactly .25 and any tiny error is compounded as the clicks increase.

[Gwion]

Thanks Brett. Data below. The scope isn't calbrated. Wouldn't know where to start. I guess measure 100yards exactly and fire/click.fire click/fire/click/measure group.... but that doesn't help if my shooting isn't spot on. It's a Sightron S-Tac.

Manufacturer: Hornady Description: A-Max®
Caliber: 0.284 in Weight: 162.0 gr
Ballistic Coefficient: 0.625 G1 (ASM)
Muzzle Velocity: 2614.0 ft/s
Sight Height: 5.40 cm Zero Range: 109yds(100m)
Zero Height: 0.0 inch
Wind Speed: 15.0 kmh
Temperature: 20.0 °C Pressure: 30.12 in Hg (1019.98 hPa)
Humidity: 40 % Altitude: 280.0 m

[Chronos]

Sight height 5.4cm?

The difference in drop could be put down to wind change, tail wind to head wind increasing flight time for example.

As for your actual drop V predicted drop its possible your chronograph is a little off but I wouldnt expect to see that much difference. Maybe go back and double check all your input data, but I'm sure you already have. The .625BC is hornady' published BC, try using Litz's G7 BC of .307 and see if that changes anything.


Chronos

[brett1868]

Forgot to ask...barrel length and twist rate..

[Gwion]

Barrel is 24" and 1:9twist.

Chronos, yeah, I double checked all input data. I measured the sight height from centre of barrel to centre of scope with my calipers; it was 54mm. I'll double check it tomorrow but I'm pretty sure that's accurate.

PS. Just have both Hornady G1 and Litz G7 BC as optional inputs. I tried both and I think the data I listed was the Litz G7.

[Strikey]

Your second string was lower than your first when shot 2hrs later? Was it warmer, sun a bit further up? Reckon a bit of mirage might have come into play, something I have learnt shooting over the blacksoil up here I have tried a few ballistic apps, can't get my head around them Nothing beats real world application such as putting bullets on paper

[brett1868]

Just ran the math through the iPad edition of Applied Ballistics and I'm getting 10.3 MOA for 500yds and 14.0 MOA for 600yds. According to "Ballistic Performance of rifle Bullets" by Mr Litz, the BC changes as expected with velocity so I punched in the various velocities and distances according to the below table. Very close to the JBM numbers but still no explanation for the big difference at 500.

FPS = G7 BC
>3000 = 0.315
3000-2500 = 0.311
2500-2000 = 0.303
2000-1500 = 0.302
<1500 = 0.305

A good article on scope calibration from the gent who produces Lex Talus Field Firing Solutions as used by many military sniper teams. I'd run your numbers through my copy but it's PDA based and it's buried in the garage under a pile of empty brass. If I get a chance tomorrow I'll dig it out and fire it up

http://www.lextalus.com/Articles/Scope%20Calibration.pdf

[Gwion]

Your second string was lower than your first when shot 2hrs later? Was it warmer, sun a bit further up? Reckon a bit of mirage might have come into play, something I have learnt shooting over the blacksoil up here I have tried a few ballistic apps, can't get my head around them Nothing beats real world application such as putting bullets on paper

Hey Strikey, first string was about 930-10am, second was just before 1200. 1st = 26C, 1015.8hPa, 90deg wind, ele 8.75; 2nd = 29C, 1015.3hPa, tail/fishtail wind, ele 9.75.

My understanding was that a tail wind was likely to result in less elevation and that warmer conditions should equate to slightly higher velocities; again less elevation, not more!?! I have plugged these figures into JBM and they have resulted in very little difference in output figures from the original test data at those ranges.

Totally agree that real world data is best, so i have created my own little data sheets for my load taking in range, temp, humidity, bar and wind sp/dir.

PS: Yes, a bit of miragee had developed by the second string.

[Gwion]

Thanks Brett. I'll read up on the scope calibration. Gathering the real-world data/dope will take some time for each load. I'm just trying to get my head around the whys and whats of these disparate figures so that i can get some semi reliable starting points for various ranges if i get the chance to shoot some strings out further at 700, 900 or 1000yd on a different club/range.

[Gwion]

E'gads! That's a process and a half. Don't know how i'll accurately measure the range over a gully. I used a string line to determine range from firing points to targets. More than adequate for target practice and sighting zeros for hunting but no good for this scope calibration due to the stretch of the string over not allowing an accurate enough measurement. By the way, i did try a few different +/- inputs for range of 1-2 meters/yards but the out puts between 300-600yards were insignificant/non-existent in the JBM output data.

Maybe i can use some of my 50lb braid from one of my fishing reels. That should have suitable low stretch values to give an pretty accurate reading of range..... Oh, the fun and the games.....

[happyhunter]

.

[Gwion]

hmmm... any suggestions of some cheap/free software that allows turret click value input?

Just been looking at a few and none seem to have it, as yet, from my searches.

[deye243]

hmmm... any suggestions of some cheap/free software that allows turret click value input?

Just been looking at a few and none seem to have it, as yet, from my searches.

this is what i use and 1st round hits are now the norm not the surprise they used to be $10 from memory


http://www.shooterapp.net/

[Gwion]

Thanks mate.

[deye243]

Thanks mate.

no problems , i use it out to 1600 yards but there's one thing it doesn't do and that's read the breeze

[chacka]

The difference in drop could be put down to wind change, tail wind to head wind increasing flight time for example.

Over 500 yards you think wind alone could be enough for such a significant difference?